The production of packing sheets on known calenders of this kind required a high degree of skill on the part of the service personnel in order to produce a packing sheet of high quality from rubber-fiber mixtures. The requirements on the service personnel have considerably increased since the replacement of asbestos fiber by other fibers in the process. Some relief has been attained through the introduction of a microprocessor controlling the operation of the calender. However, even when working with a microprocessor controlling the operation of the calender, the requirements remain high. For each packing sheet produced must have an exact predetermined thickness to the tenth of a millimeter whereby the thickness of the sheet over its entire width and its entire length remains constant. Deviation from the required thickness value is cause for the sheet to be sorted out as a second class product whereby it is very difficult to sell because with present day technical requirements first class packing sheets are required.
The required thickness measurement has heretofore been effected with the use of a dial gauge which measures the spacing between the cylinder journal of the heated cylinder and that of the counter pressure cylinder. The difference of the spacing of the cylinder journals from one another, measured during the building up of a packing sheet on the heated cylinder is the thickness of the packing sheet only inexactly because temperature changes occurring during the building up of the packing sheet materially change the cylinder diameter, in particular that of the heated cylinder because of its greater diameter, and because the forces acting on the packing sheet in the cylinder gap further falsify the measurements.
For these reasons an experimental program using a microprocessor has been recorded, the packing sheet produced has been checked and a program that has been found to produce a satisfactory packing sheet has been used for the production of other packing sheets. However other influences such, for example as changes in the ambient temperature, can lead to the production of packing sheets which are not exact reproductions of the master packing sheet.